A Metal-oxide semiconductor (MOS) image sensor typically comprises an array of picture elements (pixels), which utilizes light-sensitive MOS circuitry to convert photons into electrons. The light-sensitive MOS circuitry typically comprises a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode. Each sensor, or pixel, generates electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. The electrons are converted into a voltage signal in the pixel and further transformed into a digital signal which will be processed by an application specific integrated circuit (ASIC) or other circuitry.
A MOS image sensor, or simply a MOS sensor, may have a front side where a plurality of dielectric layers and interconnect layers are located connecting the photodiode in the substrate to peripheral circuitry, and a backside having the substrate. A MOS sensor is a front-side illuminated (FSI) image sensor if the light is from the front side of the sensor; otherwise it is a back-side illuminated (BSI) sensor with light incident on the backside. For a BSI sensor, light can hit the photodiode through a direct path without the obstructions from the dielectric layers and interconnects located at the front side, which helps to increase the number of photons converted into electrons, and makes the MOS sensor more sensitive to the light source.